The disclosure relates generally to steam turbines, and more particularly, to a system for controlling net thrust in a steam turbine to maintain thrust levels within an acceptable range of values, and avoid damage to the thrust bearing. The system may also prevent damage to an active retractable seal.
In a rotating turbomachine, thrust is an axial force acting on the rotating parts. Thrust is caused by unequal pressures acting over unequal surface areas, and changes in momentum of the fluid (steam) circulating through the machine. The sum of all axial forces acting on the rotating components of the turbine is referred to as “net thrust”. This net thrust is typically transmitted to a stationary thrust bearing which, in turn, is anchored to a foundation for the steam turbine. The thrust developed by the steam turbine has two components. First, stage thrust is thrust resulting from the pressure distribution around a stage bucket (blade), a cover, a wheel, etc. Stage thrust is usually in the direction of steam flow. Second, step thrust results from variations in the diameter of the rotating shaft to which the buckets are mounted, and the local pressure at points along the length of the steam turbine.
Conventional methods for controlling thrust in a steam turbine include: 1) using a balance piston at the high pressure (HP) section, 2) varying the rotor diameter in each section, 3) varying the number of stages comprising each section, and 4) establishing an appropriate configuration for each of the low pressure (LP), intermediate pressure (IP), and high pressure (HP) sections of the steam turbine. However, most currently available methods only control thrust under “normal” operating conditions. As an engine design is completed, and its operating conditions are fixed, the net thrust of the steam turbine is specified, and typically cannot be adjusted dynamically or actively, either under normal conditions or during extreme, perhaps fault-related, operating conditions.
There are a number of extreme operating conditions that have the potential to create large thrust forces. Examples include but are not limited to: intercept valve closed condition, a trip condition in which all steam flow stops, use of maximum pressure steam in the high pressure turbine when extraction of steam from the high pressure turbine does not dump back into the steam turbine. As a result of the above situation, thrust bearings must be sized to accommodate all of the different operating conditions, even the rare, extreme thrust operating conditions. Building a thrust bearing to address rare operating conditions increases costs and creates power losses. If the thrust bearings are not configured to accommodate all extreme thrust operating conditions, then the steam turbine cannot operate at those conditions, which may result in failure to capture an optimum value-to-cost ratio. Prior approaches to address this situation include tapping into a steam inlet bowl pressure or damping pressure to a low-pressure region to offset thrust. Unfortunately, these methods are not capable of offsetting large amounts of thrust without wasting large amounts of steam, which also greatly decreases performance.
Another challenge is protection of seals during transient conditions. To avoid rubbing of seal teeth and degradation of sealing functions during a transient, such as passing a critical speed of the rotor, seals can be retracted via a spring bias, and then closed by pressure once a steady state operating condition is reached. Most designs include a passive retractable seal, activated by available operating pressure in the system. A more advanced design is referred to as an active retractable seal (ARS), in which a bypass valve is used to actively control the opening and closing of the seal on demand. The ARS is opened as long as the turbine does not reach a stable operating condition, and closed at a time when the turbine efficiency is the concern. If a high-level vibration, over-speed or any abnormal operation is detected, the seal can be retracted instead of waiting for the system pressure drop. Therefore, seals are protected from rubbing and sustainable performance can be obtained. The ARS ring may consist of multiple arcuate segments. The open (retracting) and close may be limited to some segments while the rest is biased to close all the time.